To manufacture general alkaline storage batteries, an electrode group made up of a positive electrode, a negative electrode and a separator is housed in an outer can serving also as a negative electrode terminal, an alkaline electrolyte is injected into the outer can, and then an upper end opening of the outer can is sealed with a sealing body. This sealing body has a lid plate which is fixed to the upper end opening of the outer can via an insulating packing material, and a positive electrode terminal electrically connected to the lid plate.
In such alkaline storage batteries, the negative electrode at the outermost periphery of the electrode group is in contact with an inner wall of the outer can, so that the negative electrode and the outer can (negative electrode terminal) are electrically connected to each other. Meanwhile, the positive electrode in the electrode group is connected to one end of a positive electrode lead made of a belt-like metal. The other end of the positive electrode lead is connected to the lid plate. Consequently, the positive electrode is electrically connected to the positive electrode terminal via the positive electrode lead and the lid plate.
It is known that when such alkaline storage batteries cause an external short circuit, an excess current flows inside the batteries and heat is generated.
Accordingly, in the alkaline storage batteries, it is necessary to secure the safety of the batteries at the occurrence of the external short circuit. One of the methods to secure the safety of the batteries is known to use, for example, a positive electrode lead that incorporates a positive temperature coefficient (PTC) thermistor (see, for example, Patent Document 1). This PTC thermistor is an electronic component made of a resin containing conductive particles. The PTC thermistor has characteristics to have a low electric resistance value and good conductivity in normal conditions but to have a rapid increase in the electric resistance value when the temperature of the PTC thermistor increases and reaches a specified value.
The PTC thermistor incorporated in the positive electrode lead functions as shown below on the occasion of an external short circuit.
First, when a battery including the positive electrode lead that incorporates the PTC thermistor makes an external short circuit due to a certain cause, an excess current flows inside the battery. With the excess current flow, a large current also flows to the PTC thermistor, which causes an increase in temperature of the PTC thermistor. When the temperature of the PTC thermistor reaches a specified value, the electric resistance value of the PTC thermistor increases. Consequently, electric conduction from the electrode group to the positive electrode terminal is suppressed at a portion of the PTC thermistor in the positive electrode lead. As a result, the excess current flow inside the battery is suppressed, and heat generation is suppressed thereby.
Conventional positive electrode leads that incorporate the PTC thermistor are manufactured according to the following steps for example. First, two metal band-shaped bodies and a rectangular PTC thermistor are prepared. These band-shaped bodies are placed in series and are made to face each other at an interval so that their tip portions partially overlap with each other. The PTC thermistor is placed between the tip portions which face each other at an interval. The tip portions of these band-shaped bodies and the PTC thermistor are stacked. The upper and lower surfaces of the PTC thermistor and the tip portions of the respective band-shaped bodies which overlap with each other are joined by, for example, soldering. Accordingly, a positive electrode lead that has the PTC thermistor therein is obtained. Here, when the positive electrode lead as described above is manufactured, the PTC thermistor is soldered with the band-shaped bodies in the state of, for example, being held by a specialized tool. In this case, in order to prevent the specialized tool and the band-shaped bodies from touching each other, the band-shaped bodies having tip portions smaller than the PTC thermistor as viewed from a plane are used. Accordingly, in the obtained positive electrode lead, the area of a portion of the PTC thermistor which is not covered with the tip portions of the band-shaped bodies, i.e., the area of an exposed surface of the PTC thermistor, becomes relatively large.
The positive electrode lead that incorporates the PTC thermistor is placed between the sealing body and the electrode group in an upper space inside the can. The upper space inside the can is filled with gas atmosphere that is a mixture of both an oxygen component (high-pressure oxygen atmosphere) generated by chemical reactions at the time of charge and discharge and an alkaline component (alkaline atmosphere) derived from electrolyte inside the battery.
In this case, when the PTC thermistor is exposed to the oxygen atmosphere and the alkaline atmosphere, the function of the PTC thermistor as a positive electrode lead that can suppress current may be compromised under the influence of the oxygen component and the alkaline component. Specifically, the oxygen component in the atmosphere erodes constituent materials of the PTC thermistor, which causes the PTC thermistor to be deteriorated. Moreover, the alkaline component not only erodes general resin but also erodes a soldering portion (joined portion) where the constituent materials of the PTC thermistor and the positive electrode lead are bonded. This may cause deterioration and detachment of the PTC thermistor. Such failures attributed to the oxygen component and the alkali component tend to occur more as the exposed area of the PTC thermistor is larger.
Accordingly, to eliminate such failures, generally the PTC thermistor is sealed with a synthetic resin material so as to prevent contact with oxygen, and further a portion in the positive electrode lead that incorporates the PTC thermistor is entirely coated with a large-sized masking tape having alkali resistance so as to prevent corrosion caused by the alkaline component. Thus, measures are taken to protect the PTC thermistor from oxygen atmosphere and the alkaline atmosphere.
In the conventional positive electrode leads that incorporate the PTC thermistor as described above, the exposed area of the PTC thermistor is relatively large, so that a larger amount of the synthetic resin material is used for protective measures. Since the synthetic resin material is further covered with the masking tape, the portion that incorporates the PTC thermistor becomes relatively bulky. Accordingly, it is necessary to secure a relatively large space between the sealing body and the electrode group inside the battery where the positive electrode lead is housed. Accordingly, the batteries including the positive electrode lead that incorporates the PTC thermistor are limited to relatively large batteries of AA size or larger.